1. Field of the Invention
The invention relates to an improvement in imaging transverse flow velocities using spectral bandwidth of the Doppler frequency shift in phase-resolved optical Doppler tomography.
2. Description of the Prior Art
Optical Doppler tomography (ODT) combines Doppler velocimetry with optical coherence tomography (OCT) for noninvasive location and measurement of particle flow velocity in highly scattering media with micrometer-scale spatial resolution. The principle employed in ODT is very similar to that used in radar, sonar and medical ultrasound. ODT uses a low coherence source and optical interferometer to obtain high spatial resolution gating with a high speed scanning device such as a conventional rapid scanning optical delay line (RSOD) to perform fast ranging of microstructure and particle motion detection in biological tissues or other turbid media.
To detect the Doppler frequency shift signal induced by the moving particles, several algorithms and hardware schemes have been developed for ODT. The most straightforward method to determine the frequency shift involves the use of a small time fast Fourier transform (STFFT). However, the sensitivity of this method is mainly dependent on the FFT time window, which limits axial scanning speed and spatial resolution when measuring slowly moving blood flow in small vessels that requires high velocity sensitivity. However, a phase-resolved technique can decouple the Doppler sensitivity and spatial resolution while maintaining high axial scanning speed.
The limitation of phase-resolved ODT is that the dynamic range of the Doppler frequency shift measured is small because of an aliasing phenomenon caused by 2π ambiguity in the arctangent function. In Y. Zhao et. al., Opt. Lett. 25, 1358 (2000) on phase-resolved ODT, we reported on the use of the standard deviation of the Doppler spectrum to locate the microvasculature.